The project consists of three interlocking parts. (1) We plan to analyze the protein phenotypes of murine leukemia virus (MuLV)-infected fibroblasts, with emphasis on the polyprotein precursors to the major core and envelope proteins of virions. The sites of intracellular cleavage and glycosylation of the viral polypeptides will be determined. The relationship of the core polyprotein to a glycosylated species which we have identified on cell surfaces will be explored; the latter species contains peptide sequences of at least three major core proteins. (2) MuLV protein expression in mouse lymphoid cells will be subjected to a similar analysis. Freshly isolated normal and leukemic cells and established leukemic cell lines will be used. The presence of glycosylated core polyprotein on the cell surface will be evaluated with respect to leukemic status, virus production, and its apparent role as bearer of the GCSA. The induction of a newly characterized type-specific viral envelope protein, X-gp70, found on leukemic and pre-leukemic cells, will likewise be explored in relation to leukemogenesis, the production of endogenous virus, and the appearance of the X.1 leukemia antigen. (3) We shall apply a number of new techniques in an effort to demonstrate leukemogenic transformation by MuLV of lymphoid cells in vitro. MuLVs from spontaneous or radiation-induced leukemias will be used as transforming agents. New types of potentially susceptible cells and new assays for transformation will be employed. The protein phenotypes of transformed clones will be determined.